Creep behavior of the heusler type structure alloy Ni 2 AlTi
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I-NICKEL
ATOM
O-ALUMINUM
u
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TITANIUM
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Fig. 1--Unit cell of the HeusIer type structure~ of a0 type d i s l o c a t i o n s in m a t e r i a l deformed at t e m p e r a t u r e s above 300K. z'5 We now examine v a r i o u s p o s s i b l e glide modes in the A2BCor Heusler type s t r u c t u r e . As is the c a s e with the B2 type s t r u c t u r e the c l o s e s t packed planes a r e of the {110} type, and the a r r a n g e m e n t of A, B and C atoms within a (101) plane is shown in Fig. 2. An ins t r u c t i v e way to consider the Heusler phase is to note that it is formed from s e t s of these planes a r r a n g e d with a two l a y e r r e p e a t sequence. P o s s i b l e slip t r a n s lations between two adjacent (:~01) planes a r e indicated by the v e c t o r s in Fig. 2. F o r this d i s c u s s i o n , however, only one atom (C') in the second atomic l a y e r need be considered, this is of the C atomic specie. In an ao[010 ] s l i p t r a n s l a t i o n atom C' is d i s p l a c e d along the [010] v e c t o r shown in Fig. 2 into a position in which C atoms a r e now n e a r e s t neighbors. If the ent i r e second atomic l a y e r is c o n s i d e r e d the ao[010] VOLUME 7A, JANUARY 1976-23
m m t h i c k and they w e r e cut p e r p e n d i c u l a r to the c o m p r e s s i o n a x i s . A f t e r d i m p l i n g the d i s c s with a j e t c u t t e r t h e y w e r e e l e c t r o p o l i s h e d in a 10 p c t p e r c h l o r i c a c i d m e t h a n o l s o l u t i o n at 253 K at a p o t e n t i a l of 10 v o l t s . The f o i l s w e r e e x a m i n e d in a P h i l i p s 300 m i c r o s c o p e using a • d e g t i l t and 360 d e g r o t a t i o n g o n i o m e t e r s t a g e . A s p e c i a l c o m p u t e r p r o g r a m 6 w a s u s e d to f a c i l i t a t e the r a p i d o r i e n t a t i o n of the foil in o b t a i n i n g a l a r g e n u m b e r of t w o - b e a m c o n d i t i o n s . As w e l l a s c h a r a c t e r i z i n g d i s l o c a t i o n s b y the d e g r e e of v i s i b i l i t y o b t a i n a b l e with d i f f e r e n t c o n t r a s t conditions a c t u a l i m a g e s w e r e c o m p a r e d with c o m p u t e r s i m u l a t e d i m a g e s . The c o m p u t e r m e t h o d is s i m i l a r to that of Head 7 but u s e s a d i f f e r e n t n u m e r i c a l p r o c e d u r e which r e d u c e s the c o m p u t a t i o n t i m e . The m e t h o d t a k e s into account the effect of e l a s t i c a n i s o t r o p y and the v a l u e s of the e l a s t i c c o e f f i c i e n t s a r e t h o s e for NiA1. 8 Fig. 2--Possible slip movements in the (i01) plane of the
RESULTS
A2BC Heusler type structure. C' is a C-type atom in an adjacent (101) atomic layer.
translation is seen to create both B-B and C-C type wrong bonds; the fault thus formed is termed a sublattice antiphase boundary. Thus unit slip in (100) type directions must be accomplished by the movement of pairs of ao unit dislocations, in which the ao(100> dislocations are coupled by a
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